1. Field of the Invention
The present invention generally relates to an antenna for radiating electromagnetic waves.
2. Description of the Related Art
Satellite Digital Audio Radio Service (SDARS) providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to Earth. SDARS providers use multiple satellites in a geostationary orbit or in an inclined elliptical constellation. The elevation angle between the respective satellite and the antenna is variable depending on the location of the satellite and the location of the antenna. Within the continental United States, this elevation angle may be as low as 20 degrees. Accordingly, specifications of the SDARS providers require a relatively high gain at elevation angles as low as 20 degrees.
The automotive industry is increasingly including antennas with SDARS applications in vehicles, and specifically mounted to automotive glass. However, certain parts of the vehicle, such as a roof, may block RF signals and prevent the RF signals from reaching the antenna at certain elevation angles. Even if the roof does not block the RF signals, the roof may mitigate the RF signals, which may cause the RF signal to degrade to an unacceptable quality. When this happens, the antenna is unable to receive the RF signals at those elevation angles and the antenna is unable to maintain its intrinsic radiation pattern characteristic. Thus, antenna performance is severely affected by the roof obstructing reception of the RF signals, especially for elevation angles below 30 degrees. In order to overcome this, a radiation beam tilting technique can be used to compensate for signal mitigation caused by the vehicle body. Since antennas capable of receiving RF signals in SDARS frequency bands are typically physically smaller than those antennas receiving signals in lower frequency bands, it becomes challenging to tilt the antenna radiation main beam from the normal direction to the antenna plane, which is substantially parallel to the glass where the antenna is mounted.
One such antenna implementing a radiating beam tilting technique is disclosed in U.S. Pat. No. 7,126,539 (the '539 patent). The '539 patent discloses an antenna having a ground plane and a first dielectric layer disposed on the ground plane. A second dielectric layer having a relative permittivity different than that of the first dielectric layer is disposed adjacent to the first dielectric layer. A feeding element is embedded in the first dielectric layer adjacent to the second dielectric layer. The antenna of the '539 patent produces a directional radiation beam with a highest gain portion at a certain elevation angle. Due to the difference between the relative permittivity of the second dielectric layer compared to the first dielectric layer, the radiation beam tilts from a higher to lower elevation angle, thus tilting the highest gain portion, accordingly. However, the antenna of the '539 patent is only able to tilt the radiation beam in one direction. At lower elevation angles, the roof of the vehicle causes too much signal mitigation.
Although the antennas of the prior art may receive a relatively high gain at relatively low elevation angles, an antenna is needed for SDARS applications that provides a radiation beam with omnidirectionality at lower elevation angles when mounted on a tilted pane (i.e., a window) of a vehicle while maintaining acceptable gain, polarization, and directionality properties.